The extraction of iso-olefins is accomplished industrially by exploiting the preferential reactivity of iso-olefins with methanol to form the corresponding methyl ether compounds. After the ethers are isolated by distillation, the pure iso-olefin can be regenerated by decomposition of the ether over acid catalysts at elevated temperature. For example, isobutylene can be extracted from mixed C.sub.4 streams by preferential formation of methyl tertiary butyl ether (MTBE), distilling off the remaining light C.sub.4 materials, and finally cracking the MTBE back to methanol and isobutylene. Similarly, isoamylene can be extracted from C.sub.5 streams by formation of tertiary amyl methyl ether followed by distillation and crackback.
In each of the above examples the presence of diolefins in the mixed streams causes operational problems. Diolefins as well as olefins are produced in steam cracking and/or coking and are often present in small quantities in the mixed stream after removal of the majority of the diolefins in prior processing, e.g., DMF (dimethylformamide) extraction. Such diolefins also readily react with methanol resulting in methyl ether compounds containing one double bond (unsaturated ethers). Methyl ether streams containing even low levels of unsaturated ether will cause rapid fouling of the ether decomposition catalyst. Typically, this problem is solved by selective hydrogenation of the dienes before the stream enters the etherification reactor.
This approach has several drawbacks. Foremost, over-hydrogenation must be avoided. This is not easily accomplished since there are normally very low levels of dienes (1-3%) in the presence of mono-olefins. Overhydrogenation would not only lead to loss of desired mono-olefin product and hydrogen but, inasmuch as hydrogenations are exothermic, this situation is a potentially very dangerous one. Additionally, C.sub.4 or C.sub.5 olefin streams usually contain relatively large amounts of mercaptans. Mercaptans are known noble metal catalyst poisons. All of these factors combined necessitate that a very selective and rugged hydrogenation catalyst be employed. Lastly, treatment of the dienes at this stage requires that the entire olefin containing stream pass through the hydrogenation reactor. This requires the use of a very large and therefore expensive reactor.